The present invention relates to a fabrication method of a liquid crystal display device, and particularly relates to a fabrication method of a liquid crystal display device of a pixel-on-passivation (POP) structure.
Since an aperture ratio of unit pixels composing a liquid crystal display (LCD) device directly relates to brightness of a display per se, a high aperture ratio of an LCD device has been sought for conventionally. One example of means to achieve this is to make an LCD device in a POP structure as follows: as shown in FIG. 10(c), an interlayer insulating film 104 is provided between a glass substrate 101 having active elements (switching elements) such as TFT (thin film transistors) 102 and pixel electrodes 103b (indicated by alternate long and short dash lines in FIG. 10(a)), and each pixel electrode 103b is connected with each lower layer electrode 105 as a transparent electrode made of ITO (indium tin oxide) or the like via a contact hole 106. In the case of the LCD device of the foregoing POP structure, regions extending to above the signal lines shown in FIG. 10(a) (gate signal lines 122 and source signal lines 121) can be used as pixel regions. Accordingly, this device has a higher aperture ratio as compared with that of a non-POP-structure LCD device shown in FIG. 10(b) having a pixel electrode 103a (indicated by alternate long and two-short dashes lines in FIG. 10(a)).
Incidentally, FIG. 10(a) is a plan view illustrating a region on a pixel substrate (back substrate) corresponding to one pixel and its surroundings, the pixel substrate being a substrate on which pixel electrodes are provided. The pixel substrate is a substrate on which pixels are formed. The figure illustrates the pixel electrode 103a of the non-POP structure and the pixel electrode 103b of the POP structure together, for comparison.
In order that the pixel electrode 103b should have a light diffusing property, fine projections and recesses are formed on a surface of the interlayer insulating film 104 as shown in FIGS. 11(a) and 11(b), and moreover, the pixel electrode 103b is formed as a reflection electrode by using a high-reflection material such as aluminum. Consequently, an LCD device of reflection type having a high aperture ratio and not undergoing parallax can be realized.
Incidentally, FIG. 11(a) is a plan view of a pixel substrate provided with reflection electrodes on which projections and recesses are formed (contact holes are not shown), while FIG. 11(b) is a cross-sectional view of a region of the pixel substrate corresponding to one pixel.
Furthermore, a hybrid-type LCD device as shown in FIGS. 12(a) and 12(b), that is capable of reflection-type display and transmission-type display both, has been also developed. To form the foregoing LCD device, projections/recesses regions (reflection regions) 107 and regions (transmission regions) 108 from which the interlayer insulating film 104 is removed are simultaneously formed, so that, in the pixel electrode 103b, a high-reflection material such as aluminum is applied in the reflection regions 107, while the lower layer electrodes 105 functioning as the transparent electrodes are used as transmission regions 108.
The foregoing interlayer insulating film 104 is required to possess the following characteristics:
(i) a sufficient film thickness;
(ii) a small variation of the film thickness in one substrate; and
(iii) good processibility.
Examples of the such interlayer insulating film 104 include an inorganic film made of SiNx or SiO2, and a photosensitive organic film (photosensitive resin) such as a photoresist, but the inorganic film made of SiNx or SiO2 is difficult to be formed thick and to be processed. Therefore, it is substantially impossible to adopt the inorganic film in a reflection-type LCD device that requires shape-regulated fine projections and recesses to obtain a desired light diffusing property.
On the other hand, since the contact holes 106 and recessions and processions can be formed with respect to the foregoing photosensitive organic film by photolithography process, the photosensitive organic film is often adopted as the interlayer insulating film 104 in the LCD device of the POP structure.
However, by the foregoing conventional method, problems mentioned below arise as to (1) layer thickness distribution of the interlayer insulating film, (2) paralytic capacitance, (3) fabrication of a reflection-type LCD device, and (4) fabrication of a hybrid-type LCD device.
(1) Layer Thickness Distribution of Interlayer Insulating Film
FIGS. 13(a) through 13(e) illustrate a typical fabrication process of an LCD device of the POP structure. Note that TFTs and signal lines are omitted in FIGS. 13(a) through 13(e), so as to avoid complexity of illustration. The method of fabricating an LCD device of the POP structure is as follows.
(i) A photosensitive resin film is formed by spin-coating as the interlayer insulating films 104 on the glass substrate 101 on which the lower layer electrodes 105 are formed(see FIG. 13(a)).
(ii) The foregoing photosensitive resin film is exposed so as to be formed into the interlayer insulating films 104 by means of a photo-mask 110, so that the contact holes 106 for connecting the foregoing lower layer electrodes 105 and the pixel electrodes 103b that will be formed by a later step (see FIG. 13(b)).
(iii) The interlayer insulating film 104 is completed through development and baking (see FIG. 13(c))
(iv) An ITO film for formation of the pixel electrodes 103b is formed on the foregoing interlayer insulating film 104 (see FIG. 13(d)).
(v) The pixel electrodes 103b are formed by shaping the foregoing ITO film into a predetermined shape (see FIG. 13(e)).
In the foregoing step (i) shown in FIG. 13(a), the interlayer insulating film 104 is formed by spin-coating. The advantage of spin coating is that it allows a relatively uniform film thickness to be easily obtained. But, since a photosensitive resin material containing a solvent is applied, a phenomenon like xe2x80x9cdried state irregularityxe2x80x9d occurs when the solvent vaporizes. This phenomenon of xe2x80x9cdried state irregularityxe2x80x9d becomes more remarkable as the interlayer insulating film 104 is thicker.
Furthermore, according to the operational principle of the spin coater, the film thickness of a peripheral interlayer insulating film 104a on the periphery of the substrate tends to become thicker than the film thickness of a central interlayer insulating film 104b on a central part of the substrate as shown in FIG. 14(a), due to influences of surface tension and the like. Consequently, layer thickness distribution occurs to a certain extent in a single substrate.
Descriptions about effects achieved by thickening an interlayer insulating film are omitted here since the xe2x80x9cParalytic Capacitancexe2x80x9d section, below, will mention the same, but the following three methods are deemed applicable to laminate a material of the interlayer insulating film or the like thicker by spin coating:
(i) to decrease the rate of rotation of the coater;
(ii) to apply the material repeatedly; and
(iii) to increase the viscosity of the material to be applied.
In the case of a coating process using a spin coater, a uniform film thickness is achieved by rotating a substrate, while a solvent mixed in a material is vaporized. Usually, greater effects are obtained as the substrate is rotated at a higher rate. Therefore, in the case where a film is formed by rotating the same at a lower rate, the effects decrease. For this reason, it is difficult to apply the foregoing method (i) to the process for fabricating an LCD device such as the LCD device of the POP structure typically in which xe2x80x9cthe material to be applied remains in the LCD device at the final stagexe2x80x9d.
Problems of the method (ii) are eloquent: repetition of the sequence of xe2x80x9ccoating-photolithography-bakingxe2x80x9d leads to a meaningless increase in the number of processing steps, and hence, to a decrease in the throughput and an increase in defects. In the case where the coating step is simply repeated, that is, in the case where the process is like xe2x80x9ccoating-coating- . . . -coating-photolithography-bakingxe2x80x9d, a material for the next step is spouted out before a material such as a resist is baked, and this causes the previously formed film to be melted due to a solvent of the material. Consequently, let a film formed by the first coating step have a thickness of a xcexcm, repeating the coating step n times does not allow the thickness to become axc3x97n xcexcm (the thickness becomes not more than axc3x97n xcexcm). Furthermore, irregularity in coating also tends to occur.
The method (iii) is a widely applied method for forming a film thicker. In the case of this method, however, there arise problems such as (a) an increase in the material spouting time, and (b) difficulty in optimization of various conditions of the coating process.
The following description will explain the foregoing problem (a). In the case where a material such as resist is spouted out, the material is usually filtered so that foreign materials mixed therein are removed. Here, in the case where the material to be spouted out has a high viscosity, the filtering requires a significant pressure, and also much time. If a filter of large mesh is used so that the spouting time is reduced, foreign materials cannot be perfectly removed. Therefore, use of a filter of large mesh for reducing the spouting time is not appropriate.
Moreover, the foregoing problem (b), though varying with materials and substrates subjected to the coating process, includes the difficulty in spreading the material spouted out throughout the substrate (some areas tend to remain uncoated), and the necessity of carrying out so-called xe2x80x9cconditioningxe2x80x9d significantly strictly at every stage of the coating process, such as the finishing at the final stage for making the film thickness uniform throughout the substrate. Even in the case where the optimum conditions are found, the xe2x80x9coptimum conditionsxe2x80x9d may vary with changes in the stage of the substrate, ambient conditions, changes in materials as aging, etc. Therefore, it is extremely difficult to always form an interlayer insulating film so as to be thick and to have stable properties (thickness, distribution in one substrate).
The reason why the foregoing problems relating to the film thickening have been hardly focused is that layer thickness distribution of the resist film does not particularly matter when an LCD device having resolution at a conventional level is produced, since a resist film for formation of active elements such as TFTs for an LCD device is peeled off after etching. In the case where, however, the resist film is adopted as an interlayer insulating film in the LCD device of the POP structure, the layer thickness distribution remains in the cell. Therefore, in the case of the LCD device of the POP structure, the layer thickness distribution of an interlayer insulating film directly leads to defects in cell thickness.
FIG. 14(b) schematically illustrates a cross section of a panel formed by causing (1) a pixel substrate 111 composed of the glass substrate 101 and the interlayer insulating film 104 formed thereon and (2) a counter substrate 112 to adhere to each other by means of a substrate sealing material 113, in the case where layer thickness distribution of the interlayer insulating film 104 occurs throughout the substrate. A cell gap d3 in the center of such a liquid crystal cell becomes greater than a cell gap d4 in the periphery thereof. Such non-uniformness of the cell thickness (serious cell thickness distribution in the substrate) is remarkable in a reflection-type LCD device particularly. The reasons are that adjustment by backlight as in a transmission-type LCD device is impossible since it utilizes light in the surroundings, that the panel is affected twice that in the case of the transmission-type LCD device since retardation is proportional to twice the cell thickness, etc.
(2) Parasitic Capacitance
As shown in FIG. 15, in the case where a region extending to borders corresponding to the signal lines (the gate signal lines 122 and the source signal lines 121) is utilized for display in a transmission-type LCD device of the POP structure, a region 114 is naturally produced in which the pixel electrode 103b (indicated by the alternate long and short dash lines) is superimposed on the signal lines (the gate signal lines 122 and the source signal lines 121). Furthermore, in a reflection-type LCD device, such a superimposition region 114 (hatched region) has a greater width, since light from the front is also utilized thereby allowing the gate signal lines 122 and the source signal lines 121 to be used as a part of the pixel electrode 103b. 
According to the region 114, a capacitance component called as parasitic capacitance is generated. The parasitic capacitance naturally increases as the region 114 in which the pixel electrode 103b is superimposed on the signal lines (the gate signal lines 122 and the source signal lines 121) is expanded, as expressed by the follow formula (1):
C=∈rxc2x7∈oxc2x7S/dxe2x80x83xe2x80x83(1)
where C represents parasitic capacitance, ∈r represents a dielectric constant, ∈o represents a dielectric constant in vacuum, S represents an area of the region of superimposition of the signal lines and the pixel electrode, and d represents a distance between electrodes.
A parasitic capacitance as above raises a problem of generation of cross-talk, loads on a driver, etc. Though it is possible to design a driver and an active element that allow the generation of a parasitic capacitance as above to be ignored, an increase in consumed electric power is induced in such a case, thereby impairing the advantage of low power consumption of the LCD device.
Considering the foregoing problems, it is necessary to reduce the parasitic capacitance. To reduce the parasitic capacitance, referring to the foregoing formula (1), the following means can be suggested: (i) to reduce the area S of the region of superimposition of the signal lines and the pixel electrode; (ii) to lower the dielectric constant ∈r of the interlayer insulating film; and (iii) to widen the distance d between electrodes in the region of superimposition of the signal lines and the pixel electrode. However, the area S cannot be reduced so that the aperture ratio should not fall, and it is difficult to drastically lower the dielectric constant of an organic film with respect to that of the liquid crystal. Accordingly, to widen the distance d between electrodes is most effective.
Results of experiments conducted by the inventors of the present invention prove that, in the case where an organic film having a thickness substantially equal to that of the liquid crystal is used as the interlayer insulating film, the organic film having a thickness of not less than 3 xcexcm or 4 xcexcm would not adversely affect the display (does not cause cross-talk), while an increase in consumed power can be avoided. It is, however, very difficult to apply, by using the aforementioned spin coater, a film to a thickness of not less than 3 xcexcm or 4 xcexcm while not causing layer thickness distribution that would adversely affect the cell thickness of the liquid crystal. Besides, reduction of a film thickness, called xe2x80x9cfilm thinningxe2x80x9d, that inevitably occurs in the development process and the like tends to increase the layer thickness distribution. Such layer thickness distribution also causes irregularity in light diffusion, called xe2x80x9creflection irregularityxe2x80x9d, particularly in the case where a reflection-type LCD device is produced, resulting in extreme deterioration of display performance.
(3) Fabrication of Reflection-Type LCD Device
A reflection-type LCD device utilizes light from the panel front in display by reflecting the light by means of reflection electrodes. In so doing, xe2x80x9cwhitexe2x80x9d display is carried out by utilizing a light diffusing function that is imparted to reflection electrodes. To do so, the interlayer insulating film surface is processed so as to have a pattern of projections and recesses. FIGS. 16(a) through 16(e) illustrate a process for forming projections and recesses on the surface of the interlayer insulating film.
(i) A surface of a glass substrate 101 on which the lower layer electrodes 105 are provided is coated with a photosensitive resin by spin coating, so that the interlayer insulating film 104 is formed with the photosensitive resin (see FIG. 16(a)).
(ii) An interlayer insulating film 104 is subjected to half-exposure by using a projection/recess-forming photo-mask 115 (see FIG. 16(b)).
(iii) Portions where the contact holes 106 are to be formed are exposed by using a contact-hole-forming photo-mask 110 (see FIG. 16(c)).
(iv) Exposed portions are removed by development (see FIG. 16(d)).
(v) Baking causes the interlayer insulating film 104 to be rid of sharp corners by heat, thereby making projections and recesses smoother (see FIG. 16(e)).
In the foregoing step (ii), to simplifying the process, projections and recesses on the surface of the interlayer insulating film 104 are formed by xe2x80x9chalf-exposurexe2x80x9d process. The xe2x80x9chalf-exposurexe2x80x9d process indicates a process of exposing a film made of a photosensitive resin so that, when the development process is completed, the film will still remain to some extent in regions where the resin should be removed, or in other words, so that the underneath of the photosensitive resin film will not be exposed. By adopting the foregoing half-exposure process, the interlayer insulating film 104 will have a cross section as shown in FIG. 16(d) at the completion of development, and this makes it easier to form smooth projections and recesses by the subsequent baking. However, by adopting the foregoing process while forming the interlayer insulating film 104 thicker, the following problems arise.
(a) Since the part remaining after the development is thick under the projections and recesses, the film is flatten by baking (see FIG. 17(a)).
(b) Projections and recesses can be realized to some extent by intensifying exposing light, but the flattening occurs since the projections and recesses become smoother due to the baking. Consequently, the film thickness decreases (see FIG. 17(b)).
(c) Assume a case where, to suppress occurrence of parasitic capacitance, the interlayer insulating film 104 is let to remain on the signal lines (the gate signal lines 122 and the source signal lines 121), while patterns of projections and recesses are not provided on its surface. In this case, regions having the projections/recesses-formed patterns and regions having the signal lines differ in height (see FIG. 17(c)). As a result, the cell thickness in pixel regions increases, switching domains are produced, and other problems arise.
As described above, it is very difficult to establish a process to satisfy both the xe2x80x9chalf-exposurexe2x80x9d process and the thickening of the interlayer insulating film, as well as margins in the process are narrow.
(4) Fabrication of Hybrid-Type LCD Device
FIG. 18(a) illustrates a basic structure of a hybrid-type LCD device. In the foregoing hybrid-type LCD device, retardation R1 of a liquid crystal layer 118 of a transmission region 108, and retardation R2 of a liquid crystal layer 118 of a reflection region 107 are obtained by the following formulae:
R1=xcex94nxc2x7d1xe2x80x83xe2x80x83(2)
R2=xcex94nxc2x72xc2x7d2xe2x80x83xe2x80x83(3)
where R1 represents retardation in the transmission region, R2 represents retardation in the reflection region, xcex94n represents refractivity anisotropy, d1 represents a cell thickness in the transmission region, and d2 represents a cell thickness in the reflection region.
The retardations R1 and R2 in the respective regions vary with the cell thicknesses d1 and d2, respectively, as shown in the foregoing formulae (2) and (3). In other words, the voltage-transmissivity (reflectance) characteristics vary with the cell thicknesses d1 and d2, respectively. Incidentally, FIG. 18(a) illustrates the hybrid-type LCD device in a normally black mode. In FIG. 18(a), 116 is a polarizing element, and 117 is a backlight.
FIG. 18(b) illustrates voltage-transmissivity characteristic of the transmission region 108. The voltage-reflectance characteristic in the reflection region 107 has greater variation of retardation as compared with that in the transmission region 108, as shown in FIG. 18(c), in the case where the cell thickness d1 in the transmission region 108 satisfies the following relationship with respect to the cell thickness d2 in the reflection region 107:
d1 less than 2xc2x7d2xe2x80x83xe2x80x83(4)
On the other hand, the voltage-reflectance characteristic in the reflection region 107 is substantially identical to that in the transmission region 108, as shown in FIG. 18(d), in the case where the cell thickness d1 in the transmission region 108 satisfies the following relationship with respect to the cell thickness d2 in the reflection region 107:
d1=2xc2x7d2xe2x80x83xe2x80x83(5)
For this reason, the interlayer insulating film 104 of the reflection region 107 is formed substantially as thick as the cell thickness d2, and the cell thickness d1 of the transmission region 108 is set to twice the cell thickness d2 of the reflection region 107. This allows optical characteristics to become consistent. However, considering the liquid crystal cell thickness is about 3 to 5 xcexcm, the interlayer insulating film 104 need be formed thick. This however cannot be achieved by spin coating as described above.
The object of the present invention is to provide an LCD device fabrication method that decreases film thickness distribution in the interlayer insulating film thereby allowing a stable cell thickness to be obtained, and further, that facilitates to thicken the interlayer insulating film.
To achieve the foregoing object, an LCD device fabrication method in accordance with the present invention is a fabrication method of liquid crystal display device having a pair of substrates which are provided vis-a-vis each other and between which a liquid crystal layer is provided, at least one of the substrates being a transparent substrate having transmissivity, the other substrate that is a back substrate provided vis-a-vis the transparent substrate being provided with pixel electrodes with an interlayer insulating film intervening the back substrate and the pixel electrodes, pixel electrodes being respectively connected with lower layer electrodes through contact holes formed in the interlayer insulating film, and the method is characterized by including the steps of (a) forming signal lines and the lower layer electrodes on the back substrate, (b) forming an interlayer insulating film on the back substrate by using a dry film resist, (c) forming contact holes at positions corresponding to the lower layer electrodes by patterning the interlayer insulating film to a predetermined pattern, and (d) forming pixel electrodes on the interlayer insulating film.
According to the foregoing method, in fabrication of an LCD device of the POP structure in which an interlayer insulating film is formed between pixel electrodes and a back substrate on which the pixel electrodes are provided, the interlayer insulating film is formed with a dry film resist. Therefore, it is possible to easily form an interlayer insulating film that has very small film thickness distribution inside the face of the back substrate, as well as that is formed thick. This is because that an interlayer insulating film is formed with use of a film having a uniform and desiredly great thickness.
Furthermore, for example, in the case where a conventional spin coater is used to form the interlayer insulating film, most of the resist dropped first time is spilt by centrifugal force to outside the substrate, thereby resulting in that a very small amount of the same actually remains on the substrate, but application of a dry film resist in forming the interlayer insulating film as in the present invention enables reduction of wasted materials, thereby ensuring that an increase in costs should be suppressed.
This further enables to decrease display defects that occur due to irregularities in the cell thickness. Furthermore, costs can be reduced since materials can be saved as compared with the case where the spin coater is used.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.